In regard to countermeasures taken against the outflow of HCs in a system for exhaust gas purification disposed in an internal combustion engine mounted in an automobile, a material having a microporous structure, such as zeolite, has been used, the material being known to have a peculiar characteristic of adsorbing HC species, ammonia, and the like having a large molecular diameter on micropores of the material. Such a material adsorbs HC species and the like at a temperature of 250° C. and below, but releases adsorbed HC species and the like at a higher temperature of 250° C. to 300° C. At a temperature of 300° C. and above, HC species and the like are oxidized by a catalyst supporting a precious metal, such as an oxidation catalyst (“DOC”), a three-way catalyst (“TWC”), a NOx storage reduction catalyst (“LNT”), and a NOx catalyst of the selective reduction type. The release of the HC species and the like is considered to be due to excitation of the molecules at a high temperature range.
In the case of a system for exhaust gas purification including this material as a HC adsorption material disposed in the system, the HC adsorption material ends up releasing HCs at a temperature of 250° C. and above. For this reason, if the temperature range of HC release is not set to overlap the HC light-off temperature ranges of the oxidation catalyst and the like, HCs flow out to the downstream side of these exhaust gas treatment devices, less purified exhaust gas results.
Especially, as a catalyst system which requires a countermeasure against the outflow of HCs, there are a three-way catalyst and a NOx storage reduction catalyst. In a system for exhaust gas purification employing a three-way catalyst, when the amount of fuel is increased during acceleration or the like, the air-fuel ratio becomes deeply rich; therefore, the amount of HCs flowing out increases. In the meantime, in a system for exhaust gas purification employing a NOx storage reduction catalyst, at the time of regenerating process for restoring the NOx storage capacity, it is necessary to achieve a rich atmosphere by supplying HCs or the like by post injection or by direction injection into an exhaust pipe. Accordingly, if the optimal control for the amount of NOx released from the NOx storage reduction catalyst is not performed, there arises a concern that the amount of HCs flowing out into the atmosphere may increase. Moreover, similarly to a three-way catalyst, such a concern becomes more critical during acceleration. In other words, the catalyst temperature of the NOx storage reduction catalyst rises with acceleration, and then stored NOx is released, thereby increasing occasions to perform NOx regeneration control to reduce the NOx thus released; thus, the amount of HCs flowing out is likely to increase.
As a preventive measure against the outflow of HCs, disposition of a HC adsorption material on the downstream side (subsequent stage) of a three-way catalyst or a NOx storage reduction catalyst is effective to some degree. However, the temperature range allowing HC adsorption of the HC adsorption material is a relatively low of 250° C. and below. Accordingly, when the temperature of the exhaust gas rapidly rises at such time as acceleration, there arises a problem that the effect is insignificant against the prevention of the HC outflow.
For this reason, in these systems for exhaust gas purification, as a countermeasure for the problem of the HCs flowing out during acceleration, a control for preventing an increase of the HCs flowing out at the expense of the NOx purification efficiency is carried out. For example, in the case of a three-way catalyst, the amount of fuel injection is suppressed so as to avoid leading to an excessively rich atmosphere. In the meantime, in the case of a NOx storage reduction catalyst, the air-fuel ratio rich control for the purpose of restoring the NOx storage capacity is stopped until the catalyst temperature rises and thereby the HC purification efficiency is improved.
On the other hand, as described in Japanese patent application Kokai publication No. hei 09-85078, for example, an apparatus for purifying the exhaust gas of engines is proposed as follows. In regard to the arrangement of a three-way catalyst and a HC adsorbing purification catalyst, the three-way catalyst is deposited in an upstream part of an exhaust system located inside of an engine compartment, while the HC adsorbing purification catalyst mainly including a HC adsorption agent, an oxygen supplying agent, and the like, is deposited in a downstream part of the exhaust system located in an underfloor part outside of the engine compartment. The three-way catalyst is activated in an early stage after the starting of the engine, and the HC adsorption agent is prevented from being heated up to the HC desorption temperature before this activation by use of wind received by a running vehicle for cooling.
Further, as described in Japanese patent application Kokai publication No. hei 11-210446, for example, an apparatus for exhaust purification in an internal combustion engine is proposed as follows. A lean NOx catalyst (NOx storage reduction catalyst) is arranged on the upstream side of a HC adsorption catalyst having a three-way catalyst layer deposited as an upper layer on a HC adsorption material. The temperature of the exhaust introduced into the HC adsorption catalyst is proactively decreased by disposing cooling fins located inside and outside of a pipe for cooling exhaust, a heat mass (heat capacity) made of a honeycomb structure located inside the pipe, and the like, between the lean NOx catalyst and the HC adsorption catalyst, and by cooling with a refrigerant, such as water, so as to reliably delay the HC adsorption material heated up to the desorption initiating temperature with respect to the activation of the lean NOx catalyst.
However, in the former, depending on the running speed, the temperature of the HC adsorbing agent catalyst deposited in the underfloor part varies; therefore, there arises a problem of difficulty in controlling the HC adsorption capacity. In the latter, since no NOx catalyst of the selective reduction type is provided on the downstream side, there is a problem that HCs desorbed from the HC adsorption material flows out. In addition, these three-way catalyst and HC adsorbing purification catalyst, and the lean NOx catalyst and HC adsorption catalyst, are independently formed and arranged away from each other. Accordingly, there is a problem that not only the system for exhaust gas purification becomes long but also installation of the system takes time as it has two parts of the device.